Composite photovoltaic device with parabolic collector and different solar cells

ABSTRACT

Proposed is a composite photovoltaic device with parabolic collector and different solar cells, wherein the high photoelectric conversion efficiency is achieved along with significant material cost reduction. The device comprises two or three solar cells formed on opposite sides of a transparent substrate, and a parabolic collector attached to the back side of the substrate. First thin film solar cell formed on the front side receives and converts to electricity a short-wavelength portion of the incoming Sun radiation, and transmits the long-wavelength portion. A second solar cell receives and converts to electricity a concentrated long-wavelength portion of the Sun radiation, which is re-directed toward a focal point by the parabolic collector. In one embodiment a third solar cell is included for converting an IR portion of the radiation. Thus, each solar cell utilizes a favorable part of the Sun spectrum, which allows for an enhancement of photoelectric efficiency and significant material cost reduction.

FIELD OF THE INVENTION

The invention relates to a composite photovoltaic (PV) device thatcomprises a common transparent substrate, at least two solar cells ofdifferent materials and different sizes, and concentrating paraboliccollector for providing concentrated radiation for at least one solarcell. More specifically, the invention relates to a spectrum-splittingphotovoltaic device, wherein different solar cells absorb and convert toelectricity different spectral portions of the Sun radiation, includingconcentrated portions, thus providing a significantly better utilizationof the Sun radiation, higher photoelectric conversion efficiency andsignificant reduction of photo-active materials.

BACKGROUND

At the present time one of the main problems of a global solar energyimplementation is difficulty in reaching high photoelectric energyconversion efficiency (ECE) at low cost of materials and processesinvolved in building solar cells and panels. Typically, usage of lowcost materials, such as thin films, results in lower ECE, whileachieving higher ECE requires expensive materials, e.g., monocrystallineSilicon (Si), III-V compound materials, such as InGaAs, GaAlAs, and thelikes. Some of well known techniques for overcoming high cost of III-Vcompound materials include using concentrated Sun radiation, which isdirected toward smaller solar cells to generate electrical power. Suchphotovoltaic devices (hereinafter referred to as CPV devices) allow forsignificantly lower consumption of expensive photovoltaic materials,while providing the same or higher amount of electricity. At the sametime CPV devices require additional optical elements (lenses, mirrors)and efficient cooling tools for maintaining optimal PV operation.

In order to increase ECE of CPV devices it is common to use PV structureof stacked solar cells (SC), wherein a material band gap decreases fromcell to cell in the direction away from the light-receiving surface,thus providing a multijunction solar cell (MJSC). The combination ofMJSC and concentrated radiation is described in many references, seee.g. “Current status of III-V concentrating multijunction manufacturingtechnology and device technology development” by F. Newman, D. Aiken etal., Proceedings of 23^(rd) European Photovoltaic Solar EnergyConference, Valencia, Spain, (2008). Most commonly used III-Vmaterial-based MJSC may also include low band gap layers of a singlesemiconductor, such as Ge, as described, e.g., in the Patent ApplicationTW200933913., published on Aug. 1, 2011, authors Aiken Daniel J et al.

The multijunction approach, described above, is also widely used in avariety of Si and Thin Film (TF)-based PV devices, in which caserespective PV devices are commonly called Tandem Solar Cells (TSC). ATSC device typically comprises top SC and bottom SC, each having it'sown band gap, thus providing a sunlight spectrum split between thecells. Some examples of TSC can be found, e.g., in “Tandem photovoltaicdevice and method of manufacturing the same”, Patent ApplicationCN102237417, published on Nov. 9, 2011, author Seung-Yeop Myong, and inmany other references. A TSC may operate at regular (unconcentrated) orCPV conditions.

It should be noted that, regardless of operating TSC in a regular or CPVmode, the areas of top SC and bottom SC in a TSC structure areessentially the same (defined by a “stack” dimensions), and a serieselectrical connection should be provided between top SC and bottom SC.It is, therefore, understood that, in order to form top and bottomcells, all known TSC use respective materials for top cell and bottomcell in the amounts necessary to cover the entire area of a TSC.

CPV devices of all kinds include means for concentrating incomingsunlight, these means are hereinafter referred to as solar collectors.Solar collectors may comprise set of lens, concave spherical mirrors andparabolic mirrors (the latter is hereinafter referred to asconcentrating parabolic collector or simply parabolic collector).Relevant to the present invention is the observation that, to the bestof our knowledge, all known PV devices and CPV devices utilize eitherregular (unconcentrated) Sun radiation or concentrated Sun radiation foran entire device, i.e., without splitting sunlight spectrum into theregular and concentrated portions.

SUMMARY OF THE INVENTION

The present invention provides a high-efficient composite photovoltaicdevice (hereinafter referred to as “PV device”) comprising a transparentsubstrate having a front side and a back side, a concentrating paraboliccollector attached to the back side of the substrate, and furthercomprising two or more solar cells made of different materials andhaving different sizes, formed on the opposite sides of the transparentsubstrate. The invention allows for a substantial increase of thephotoelectric energy conversion efficiency (ECE) and significantreduction of the materials needed to build the PV device. The aboveadvantages result from the following: 1) using first solar cell of thePV device to generate an electrical power by photoelectric conversion ofa first (short-wavelength) portion of the Sun radiation, 2) usingparabolic collector for concentrating and directing toward second solarcell a second (longer wavelength) portion of the Sun radiation; 3) usingsecond solar cell (or second and third solar cells, according to anotheraspect of the invention) to generate additional electrical power byphotoelectric conversion of the second portion of the Sun radiation.

Electrical power produced by the PV device is the total of the powersproduced by the first solar cell and by the second solar cell (and, foranother aspect, by the third solar cell), which results in significantimprovement of ECE. Another advantage of the PV device is significantreduction of the materials needed to build high-efficient second andthird solar cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of the PV device of thepresent invention, comprising a parabolic collector and two solar cellsof different materials and sizes formed on the opposite sides of thetransparent substrate.

FIG. 2 is a partial view of the PV device of the present invention, thatshows boundaries and the area of an efficient light absorption in thesecond solar cell.

FIG. 3A schematically shows conductive electrodes formed on the firstsolar cell and the second solar cell of the PV device.

FIG. 3B is a top view of the PV device of FIG. 3A with the conductiveelectrodes and connection links formed on the back side of thetransparent substrate.

FIG. 4 is a top view of a PV module comprising four PV devices connectedin series and in parallel.

FIG. 5 is a cross-sectional view of an embodiment of the presentinvention that comprises three solar cells of different materials andsizes formed on the opposite sides of the transparent substrate.

FIG. 6 is a cross-sectional view of the embodiment of FIG. 5, whereinthe focal point of the parabolic collector is located at a distance fromthe front side of the transparent substrate.

FIG. 7 is a graph that schematically shows the AM1 Solar spectrum alongwith spectral split and energy distribution between different solarcells of the PV device.

DETAILED DESCRIPTION OF THE INVENTION

In general, the PV device of the present invention employs the followingfeatures: 1) splitting the incoming Sun radiation into two (or three)spectral portions; 2) concentrating the longer wavelength portion (orportions), and 3) using different solar cells for photoelectricconversion of the above spectral portions. These features areillustrated by FIG. 1, which is a schematic cross-sectional view of mainembodiment of the PV device 100 and by FIG. 5 and FIG. 6, which showanother embodiment.

In FIG. 1 reference numeral 101 designates a transparent substrate(hereinafter referred to as merely a “substrate”), which is made ofmaterial transparent to the Sun radiation (typically of AM1 type), suchas, e.g., glass plate, quartz plate or transparent plastic. Thesubstrate 101 has a front side 102 and a back side 103. The front side102 may be textured and pre-coated (not shown) for an anti-reflectionpurpose as commonly used in known PV devices. A first solar cell 110 isformed on the front side 102, covering most of the front side. The firstsolar cell has a first light-receiving surface 112, which is exposed tothe Sun radiation, as shown by the dashed arrows L1, and a first backsurface 113.

The first solar cell 110 is made of a first material and has a firstthickness, thus providing photo-active absorption and photoelectricconversion of a first portion of the Sun radiation. The rest of the Sunradiation (hereinafter referred to as a “second portion”) remainsunabsorbed in the first solar cell and is transmitted through thesubstrate, as shown by the dashed arrows marked L2. The first materialis selected, e.g., from the group of thin film materials that have aband gap in the range of, e.g., 1.5 eV to 2.0 eV and a thickness in therange of, e.g., 200 to 1000 nm. One example of such materials ishydrogenated amorphous silicon (aSi:H). It is understood, that the firstportion of the Sun radiation comprises a short-wavelength part of theSun spectrum, such as the one, shown in FIG. 7 by reference AA (FIG. 7schematically illustrates the AM1 spectrum “S” and spectral splitsbetween different solar cells of the PV device). The second portion ofthe Sun radiation comprises a longer wavelength part of the Sunspectrum, as shown in FIG. 7 by reference BB and CC A spectral boundarybetween the first portion and the second portion (i.e., the longestwavelength that can be absorbed in the first solar cell) is defined by afirst band gap of the first material.

The PV device further comprises a concentrating parabolic collector 200(hereinafter referred to as a “parabolic collector”) attached to theback side 103 of the substrate 101. The parabolic collector 200 has areflective surface 210 that faces the back side of the substrate. Thereflective surface 210 is essentially a parabolic mirror commonly usedin optical and optoelectronic devices. A focal point of the reflectivesurface is marked F in FIG. 1. As shown in FIG. 1, the reflectivesurface 210 re-directs the second portion L2 of the Sun radiation towardthe focal point F, thus ensuring concentrating of the second portion L2in the vicinity of the focal point.

Furthermore, a second solar cell 120 is formed on the back side 103 ofthe substrate between the focal point F and the reflective surface 210.The second solar cell 120 has a second light-receiving surface 122 thatfaces the reflective surface 210, and a second back surface 123 thatfaces the back side 103 of the substrate 101. The second solar cell 120is made of a second material and has a second thickness, providingphoto-active absorption and photoelectric conversion of the secondportion L2 of the Sun radiation (in the embodiment of FIG. 5 and FIG. 6the second solar cell is made to absorb a part of the second portion L2,as will be described below). The second back surface 123 and the backside 103 may carry pre-deposited conductive electrodes and connectionlinks, described below.

The second solar cell 120 is positioned in the vicinity of the focalpoint F in such a way, that the projection of the focal point F onto thesecond back surface 123 in a direction perpendicular to the back side103 is within the second back surface 123. This is illustrated by thereference axis O′-O′ of FIG. 1. For example, the focal point can bepositioned inside the substrate 101, and between the second back surface123 and the front side 102, as shown in FIG. 1. It is understood that,in order to receive the most of the concentrated second portion L2, thesecond solar cell should provide the second back surface 123 (and,preferably, the central area of it) to be aligned with the focal point Falong the axis O-O′. It is further understood that the secondlight-receiving surface 122 can be pre-textured and pre-coated (notshown) for anti-reflection purposes as commonly used in known PVdevices.

As shown in FIG. 1, the second light-receiving surface 122 receives aconcentrated second portion L2 of the Sun radiation, therefore, thefirst light-receiving surface of the first solar cell may besubstantially larger than the second light-receiving surface of thesecond solar cell. More specifically, an area ratio of the firstlight-receiving surface to the second light-receiving surface may be inthe range of 10 to 200, depending on the proximity of the secondlight-receiving surface to the focal point F. This important feature ofthe PV device ensures a significant reduction of the amount of thesecond material needed to form the second solar cell, i.e., asubstantial cost reduction of the device. It also applies to a thirdmaterial of a third solar cell for the embodiment described below (FIGS.5 and 6).

According to the present invention, a band gap of the first material ofthe first solar cell 110 is wider than a band gap of the second materialof the second solar cell 120. The second material may be selected, e.g.,from the group of monocrystalline or polycrystalline materials that havea band gap in the range of, e.g., 0.7 eV to 1.5 eV, and a thickness inthe range of, e.g., 50 um to 250 um. One example of the second materialis monocrystalline Silicon (c-Si). As a result, the second solar cellprovides an efficient light absorption and photo-electric conversion ofthe second portion of the Sun radiation, which comprises a longerwavelength part of the Sun spectrum, such as the one shown in FIG. 7 byreference BB. It is further understood that a certain part of the secondportion L2 (hereinafter referred to as a “third portion”) may remainunabsorbed in the second solar cell and transmitted through thesubstrate 101 toward the front side 102. The third portion comprises aninfrared (IR) part of the Sun radiation, and will be described below inreference to the embodiment of FIG. 5 and FIG. 6.

The laws of geometric optics impose certain limitations on the amount ofthe second portion L2 of the sun radiation that can be used in thesecond solar cell. These limitations are shown in FIG. 2, which is apartial view of the PV device of the present invention, that showsboundaries of an efficient light absorption in the second solar cell.For simplicity the PV device is symmetrical with the position of theorigin O′ aligned to the center of the reflective surface 210 and to thecenter of the second solar cell 120. Reference Xf (shown for the rightside only) designates the boundaries of the reflective surface, which isapproximately equal to the width of the first solar cell). Reference Amdesignates the maximum angle of reflection for the light rays of thesecond portion L2, that will be received by the second light-receivingsurface 122. Consequently, the light ray marked L2 m and the referenceXm designate boundaries of the concentrated part of the portion L2 thatwill be received and converted to electricity by the second solar cell.It is understood, that, although references Am and Xm are shown only forthe left side of the cross-sectional view, similar boundaries exist onthe right side. It is further understood, that light rays of the secondportion L2 of the Sun radiation, which hit the reflective surface 210 atthe angles smaller than Am, will not be received by the second solarcell.

Boundary position Xm depends on the geometrical parameters of the PVdevice such as parabolic factor A (which is defined by a parabolicequation of the reflective surface Y=AX²), width of the second solarcell 120 (half of which is shown in FIG. 2 by reference d), and thedistance between the focal point F and the back surface 123 of thesecond solar cell 120 designated by reference t. Calculations based onthe laws of the geometric optics result in the following equation:

Xm=½A[SQRT(Z ²+1)−Z]  (1)

tan(Am)=Z  (2)

where A is parabolic factor of the reflective surface, Z=t/d.Apparently, for Z<<1, which corresponds to the close proximity of theback surface of the second solar cell to the focal point (as compared tothe second solar cell size) the equation (1) results in Xm beingapproximately equal to Xf. It is understood that in that case the mostof the light portion L2 is utilized by the second solar cell, and theenergy losses are negligible.

For any given parameters of the PV device a ratio S of the second lightportion L2 to the concentrated part L2 m can be calculated as follows:

S=(Xm)²/(Xf)²=SQRT(Z ²+1)−Z  (3)

Example 1

d=0.5 cm, t=0.1 cm. S=0.65 (65% of the second light portion L2 isutilized by the second solar cell).

Example 2

d=0.5 cm, t=0.02 cm, S=0.9 (90% of the portion L2 utilized).

According to the present invention, the following parameters of the PVdevice can be used: width of the first solar cell—10 cm, Xf=9 cm, halfwidth of the second solar cell d=1 cm, focal point position F (relativeto the origin O′)=4.5 cm, t=0.02 cm. It should be noted that in thiscase an area ratio of the first light-receiving surface of the firstsolar cell to the second light-receiving surface of the second solarcell is approximately 100, which result in proportionate reduction ofthe amount of the second material needed to form second solar cell.Similar analysis applies to the third material of the third solar cell,which will be described below.

FIG. 3A schematically shows conductive electrodes to the first solarcell 110 formed on the light-receiving surface 112 and on the first backsurface 113 of the first solar cell. It also shows conductive electrodesto the second solar cell 120 formed on the second back surface 123 ofthe second solar cell. In FIG. 3A references numeral 111, 115 a and 115b designate conductive electrodes to the thin film first solar cellformed on the first light-receiving surface 112. A transparentconductive electrode 111 (made, e.g. of ITO) may be formed on thesurface 112 prior to forming metal electrodes 115 a and 115 b. Thetransparent electrode 111 is electrically connected to the conductiveelectrode of one polarity, e.g., to the electrode 115 a. In order toprovide the electrode 115 b of the opposite polarity (which must beisolated of the electrode 115 a), a small area of the first solar cellmay be etched off and isolated prior to forming the conductiveelectrodes 115 b, which is shown in FIG. 3 a as the hatched region 115c. Also, the electrode 115 b can be partially formed on the first backsurface of the first solar cell.

References numeral 125 a and 125 b of FIG. 3A designate conductiveelectrodes to the second solar cell 120 formed on the second backsurface 123. It is understood that in the main embodiment of the PVdevice the second solar cell 120 is formed, e.g., as a Si-based backsidesolar cell with the electrodes of both polarities formed on the secondback surface 123. Such design allows for the second light-receivingsurface to be fully exposed to the concentrated light portion L2 m(described above), which results in higher ECE of the PV device.

FIG. 3B shows the top view of the front side of the PV device of FIG. 3Awith the conductive electrodes of FIG. 3A and connection links formed onthe front side of the substrate and on the back side of the substrate.The connection links are in contact with conductive electrodes to thefirst solar cell and to the second solar cell and provide separateoutput connections for the first solar cell and for the second solarcell. In the drawing the first solar cell is shown as the hatched square110, and an inner border of the parabolic collector (i.e., an edgecontour of the reflective surface) is shown as a dashed circle 201.According to one aspect of the invention the conductive electrodes tothe first solar cell can be formed in the corners of the first solarcell, as shown by references numeral 115 a and 115 aa for one polarityand by references numeral 115 b and 115 bb for the opposite polarity ofthe electrodes. In order to provide output connections for the secondsolar cell, the conductive electrodes to the second solar cell 125 a and125 b are complemented by connection links 135 a and 135 b formed on theback side of the substrate (ref 103 of FIG. 1) and connected to theelectrodes 125 a and 125 b respectively.

It is understood that shapes, dimensions, and locations of theconductive electrodes and the connection links are shown in FIG. 3A andFIG. 3B only schematically, and any reasonable modifications are allowedprovided that these modifications do not depart from the scope of thepresent invention. It is further understood that, in order to increasethe first light-receiving surface exposed to the Sun radiation L1 (forthe first solar cell) and the second light-receiving surface exposed tothe concentrated part of the Sun radiation L2 m (for the second solarcell), linear dimensions of the conductive electrodes and links shouldbe made as small as possible (at least for their opaque parts).Materials of the conductive electrodes and connection links can beselected from the group of metals, such as, e.g., Al, AlSi, Ni, Ag, Ti,Cu and their alloys, as well known in art.

For assembling PV devices of the present invention into PV moduleadditional connection links should be formed on the front side of thesubstrate and on the back side of the substrate as schematically shownin FIG. 4, which is a top view of a PV module segment that comprisesfour PV devices connected in series and in parallel. The additionalconnection links 145 a, 145 b, 155 a, 155 b, 145 ab, and 155 ab are incontact with conductive electrodes to the first solar cell and to thesecond solar cell of FIG. 3 a and FIG. 3 b and provide separate outputconnections for the first solar cell and for the second solar cell.

The PV module of FIG. 4 comprises four individual PV devices C1, C2, C3,and C4, in which first solar cells and second solar cells of individualdevices separately connected in parallel and in series by the connectionlinks. More specifically, connection links 145 a (for simplicity shownonly on the left side of the devices C1 and C2) provide parallelconnection between first solar cells of the devices C1 and C2;connection links 145 b (shown only on the right side of the devices C3and C4) provide parallel connection between first solar cells of thedevices C3 and C4. Furthermore, the conductive link 145 ab providesseries connection of the first solar cell of the device C2 to the firstsolar cell of the device C3. References numeral 110A and 110B designateoutput terminals for a group of the first solar cells of the PV module.

Similarly, second solar cells of the devices C1 and C2 are connected inparallel by the connection links 155 a, and second solar cells of thedevices C3 and C4 are connected in parallel by the connection links 155b; conductive link 155 ab provides series connection between the secondsolar cells of the devices C2 and C3. References 120A and 120B designateoutput terminals for a group of the second solar cells of the PV module.It is understood that for achieving a highest possible ECE of PVmodules, which are assembled of PV devices of the present invention, agroup of first solar cells and a group of second solar cells must beconnected separately to respective output terminals. It is furtherunderstood that illustrated PV module is only an example, and a varietyof connection patterns can be designed to incorporate PV devices of thepresent invention into PV modules.

FIG. 5 is a cross-sectional view of another embodiment of the presentinvention that comprises three solar cells of different materials andsizes formed on the opposite sides of the transparent substrate. In thisembodiment the second solar cell is made of a second material and has asecond thickness that provide photo-active absorption and photoelectricconversion of at least a part of the second portion of the Sunradiation, and transmits a third portion of the Sun radiation throughthe transparent substrate. The third portion is concentrated by thereflective surface 210. In order to provide best conditions forutilizing concentrated second and third portions, dimensions of theparabolic collector and positions of the substrate, the second solarcell and the third solar cell are selected providing positioning of thefocal point at a distance from the second light-receiving surface of thesecond solar cell in the direction to the front side of the substrate,as shown in FIG. 5 and FIG. 6.

Additionally, the PV device of FIG. 5 comprises a recess 305 formed inthe first solar cell, which overlaps a position of the focal point F,and a third solar cell 310 formed on the front side 102 of the substrate101 and inside the recess. The recess 305 is formed by, e.g., selectiveetching and wall-insulating of a small portion of the first material ofthe first solar cell, as is well known in art. The third solar cell 310has a third light-receiving surface 302 facing the reflective surface210 of the parabolic collector 200 (also facing the second solar cell),and a third back surface 303 facing outward in the direction of theincoming Sun radiation L1. It is understood that, although the thirdsolar cell is formed to absorb and convert to electricity the third(concentrated) portion of the Sun radiation, it may absorb a small partof the incoming regular radiation L1. It is further understood that dueto substantially smaller size of the third solar cell compared to thefirst solar cell, absorption of the L1 radiation by the third solar cell(and corresponding ECE losses) is expected to be negligible

In the present embodiment of the PV device a band gap of a firstmaterial of the first solar cell is wider than a band gap of a secondmaterial of the second solar cell, and a band gap of the third materialof the second solar cell is narrower than a band gap of the secondmaterial of the second solar cell. The third solar cell 310 is made of athird material and has a third thickness, thus providing a photo-activeabsorption and photoelectric conversion of the third portion of the Sunradiation, shown in FIG. 5 by the dashed arrows marked L3. The thirdmaterial can be selected, e.g., from the group of monocrystalline thinfilm materials with a band gap in the range of, e.g., 0.3 eV to 0.7 eV,and a thickness in the range of, e.g., 100 nm to 2000 nm. One example ofthe third material is InGaAs. It is understood the third portion L3 ofthe Sun radiation essentially consists of an IR part of the Sunspectrum, such as the one shown in FIG. 7 by reference CC.

It is understood that, due to a difference of refraction indexes betweenmaterials of the second solar cell, the substrate, and the thirdmaterial of the third solar cell, direction of the light rays L3 maydeviate from the direction toward the focal point as shown in FIG. 5 andFIG. 6. It is further understood that an area ratio of the thirdlight-receiving surface 302 of the third solar cell to the secondlight-receiving surface of the second solar cell depends on the positionof the focal point relative to the light-receiving surfaces. In view ofFIG. 5 the focal point F is positioned between the second back surfaceof the second solar cell and the third light-receiving surface of thethird solar cell, which is identical to the view of FIG. 1. In thiscase, an area of the third light receiving surface is close to the areaof the second light-receiving surface. According to another aspect ofthe present invention, which is shown in FIG. 6, the focal point F ofthe parabolic collector 200 is located at a distance from the front sideof the transparent substrate, and more specifically, is positioned at adistance from the third back surface 303 in the direction to the Sunradiation. Consequently, the area of the third light-receiving surface302 of the third solar cell 310 is smaller than the area of the secondlight-receiving surface 122 of the second solar cell 120, andconcentration factor for the third solar cell is higher thanconcentration factor for the second solar cell. This important featureensures further reduction of the amount of the third material needed toform the third solar cell, i.e., a substantial cost reduction of theentire PV device.

In the embodiment of FIG. 5 and FIG. 6 conductive electrodes andconnective links for the first solar cell, the second solar cell and thethird solar cell are formed providing separate output connection forindividual solar cells. Conductive electrodes (not shown) can be formedon the third back surface 303 of the third solar cell 310 andcorresponding connection links (not shown) can be formed on the frontside 102 of the substrate 101. Similar to the previous embodiment,connection links should provide separate connections and outputterminals for the third solar cell, as was described above in referenceto the first and second solar cells (FIGS. 3A and 3B). It is understoodthat forming connection links for the third solar cell may requireadditional patterning of the first solar cell in selected small areas,which can be achieved by etching and insulation processes known in art.

The PV device of the present invention operates as follows (FIG. 1 toFIG. 4). Incoming regular (unconcentrated) Sun radiation L1 (e.g., ofAM1 spectrum) is received by the first light-receiving surface 112 ofthe first solar cell 110. Short-wavelength first portion of the L1(shown as AA in FIG. 7) is absorbed and converted to electricity in thefirst solar cell 110, providing an output electrical power P1 throughthe conductive electrodes 115 a, 115 b, 115 aa, 115 bb (FIG. 3 a andFIG. 3 b). The second longer wavelength portion L2 of the Sun radiationremains unabsorbed in the first cell and transmitted through thesubstrate 101 toward the parabolic collector 200. It is understood thatthe second portion L2 in this embodiment is a total of the spectralsegments BB and CC of FIG. 7.

The parabolic reflective surface 210 re-directs light rays of the secondportion L2 toward the focal point F. Consequently, a concentrated partL2 m (FIG. 2) of the second portion L2 is received by the secondlight-receiving surface 122 of the second solar cell 120. A properchoice of the PV device's parameters ensures that L2 m part is close tothe entire L2 portion, as described above in reference to FIG. 2. It isunderstood that due to the high concentration ratio (provided by theparabolic collector) an area of the second solar cell is substantiallysmaller than an area of the first solar cell. It is further understoodthat due to the longer wavelength spectrum of the portion L2, ECE of thesecond solar cell (made, e.g., of c-Si) can be significantly higher thanECE of the same cell at “normal” conditions, i.e., regular Sun spectrum.The second solar cell provides an output electrical power P2 by theconductive electrodes 125 a,125 b and connection links 135 a,135 b (FIG.3 a and FIG. 3 b). Since the conductive electrodes and the connectionlinks are formed separately for the first solar cell and for the secondsolar cell, an amount of electrical power P produced by the PV device istotal of P1 and P2, i.e., P=P1+P2, which allows for achieving enhancedECE. For the PV device with previously described parameters of the firstand second solar cells a total ECE is expected to be in the range of22-25%.

Further enhancement of ECE of the PV device is achieved in theembodiment shown in FIG. 5 and FIG. 6, wherein the third solar cell 310if formed and used to absorb and convert to electricity the thirdportion L3 of the Sun radiation. The third portion is shown in FIG. 7 asthe segment marked CC. It is understood that the third portion L3entirely consists of IR part of the Sun radiation. Furthermore, an ECEfor the third solar cell is high (i.e., higher than it would be at“normal” conditions) due to the favorable spectrum of the portion L3 andhigh concentration factor. An additional electrical power P3 is producedby the third solar cell; therefore, a total electrical power P of the PVdevice in this case is equal to P1+P2+P3. For the PV device with theparameters of all three solar cells described previously a total ECE isexpected to be in the range of 25-30%. It should be noted that highvalues of ECE can be achieved in parallel with a substantial reductionof the materials needed to form the second solar cell and the thirdsolar cell.

Although the invention is shown and described with reference to specificexamples, it is understood that these examples should not be construedas limiting the areas of application of the invention and that anychanges and modifications are possible provided that these changes andmodifications do not depart from the scope of the attached patentclaims. For example, in the embodiment with two solar cells the focalpoint can be positioned both inside and outside the transparentsubstrate. Front side of the substrate can be pre-textured andpre-coated for anti-reflection purposes, as well as the first and secondlight-receiving surfaces. Locations and patterns of the conductiveelectrodes and links can be defined in many different ways providingminimal resistive losses in metal lines, and minimum of shading.Conductive electrodes to the third solar cell can be partially formed onthe third light-receiving surface.

1. A composite photovoltaic device with a parabolic collector anddifferent solar cells comprising: a transparent substrate having a frontside and a back side; a first solar cell formed on the front side of thetransparent substrate, the first solar cell having a firstlight-receiving surface and a first back surface, the firstlight-receiving surface being exposed to the Sun radiation, the firstsolar cell being made of a first material and having a first thicknessproviding photo-active absorption and photoelectric conversion of afirst portion of the Sun radiation and transmitting a second portion ofthe Sun radiation; a concentrating parabolic collector attached to theback side of the transparent substrate, the parabolic collector having areflective surface facing the back side of the transparent substrate,and a focal point; a second solar cell formed on the back side of thetransparent substrate between the focal point and the reflectivesurface, the second solar cell having a second light-receiving surfacefacing the reflective surface of the parabolic collector, and a secondback surface facing the back side of the substrate; the second solarcell being made of a second material and having a second thicknessproviding photo-active absorption and photoelectric conversion of thesecond portion of the Sun radiation.
 2. The device of claim 1, wherein aband gap of the first material of the first solar cell is wider than aband gap of the second material of the second solar cell.
 3. The deviceof claim 2, wherein the first light-receiving surface of the first solarcell is larger than the second light-receiving surface of the secondsolar cell.
 4. The device of claim 3, wherein an area ratio of the firstlight-receiving surface to the second receiving surface is in the rangeof 10 to
 200. 5. The device of claim 1, further comprising a recessformed in the first solar cell, the recess overlapping a position of thefocal point.
 6. The device of claim 5, further comprising a third solarcell formed on the front side of the substrate and inside the recess,the third solar cell having a third light-receiving surface facing thereflective surface of the parabolic collector, and the third backsurface.
 7. The device of claim 1, further comprising conductiveelectrodes to the first solar cell formed on the first light-receivingsurface and on the first back surface of the first solar cell.
 8. Thedevice of claim 7, further comprising conductive electrodes to thesecond solar cell formed on the second back surface of the second solarcell.
 9. The device of claim 8, further comprising connection linksformed on the front side of the substrate and on the back side of thesubstrate, the connection links being in contact with conductiveelectrodes to the first solar cell and to the second solar cell, andproviding separate output connections for the first solar cell and forthe second solar cell.
 10. The device of claim 6, further comprisingconductive electrodes to the third solar cell formed on the third backsurface of the third solar cell.
 11. The device of claim 10, furthercomprising connection links formed on the front side of the substrate,the connection links providing output connections for the third solarcell.
 12. A composite photovoltaic device with a parabolic collector anddifferent solar cells comprising: a transparent substrate having a frontside and a back side, a first solar cell formed on the front side of thesubstrate, the first solar cell having a first light-receiving surfaceand a first back surface, the first solar cell is made of a firstmaterial and having a first thickness providing photo-active absorptionand photoelectric conversion of a first portion of the Sun radiation,and transmitting a second portion of the Sun radiation through thetransparent substrate, a concentrating parabolic collector attached tothe back side of the substrate, the parabolic collector having areflective surface and a focal point, the reflective surface directingthe second portion of the Sun radiation toward the focal point; a secondsolar cell formed on the back side of the substrate, the second solarcell having a second light-receiving surface facing the reflectivesurface, and a second back surface, the second solar cell is made of asecond material and having a second thickness providing photo-activeabsorption and photoelectric conversion of at least a part of the secondportion of the Sun radiation, and transmitting a third portion of theSun radiation through the transparent substrate; a third solar cellformed on the front side of the substrate, the third solar cell having athird light-receiving surface facing the reflective surface, and a thirdback surface, the third solar cell is made of a third material andhaving a third thickness providing photo-active absorption andphotoelectric conversion of the third portion of the Sun radiation. 13.The device of claim 12, wherein dimensions of the parabolic collectorand positions of the substrate, the second solar cell and the thirdsolar cell are selected providing positioning of the focal point at adistance from the second light-receiving surface of the second solarcell in the direction to the front side of the substrate.
 14. The deviceof claim 13, wherein a band gap of the first material of the first solarcell is wider than a band gap of the second material of the second solarcell, and a band gap of the third material of the second solar cell isnarrower than a band gap of the second material of the second solarcell.
 15. The device of claim 14, wherein the first light-reevingsurface of the first solar cell is larger than the secondlight-receiving surface of the second solar cell.
 16. The device ofclaim 12, wherein conductive electrodes and connective links for thefirst solar cell, the second solar cell and the third solar cell areformed providing separate output connection for individual solar cells.17. The device of claim 14, wherein the first material of the firstsolar cell is selected from thin film materials having a band gap in therange of 1.5 eV to 2.0 eV.
 18. The device of claim 17, wherein thesecond material of the second solar cell is selected from monocrystalline or polycrystalline materials having a band gap in the rangeof 0.7 eV to 1.5 eV.
 19. The device of claim 18, wherein the thirdmaterial of the third solar cell is selected from mono crystalline orpolycrystalline materials having a band gap in the range of 0.3 eV to0.7 eV.
 20. The device of claim 14, wherein the first material of thefirst solar cell is hydrogenated amorphous silicon (aSi:H), the secondmaterial of the second solar cell is monocrystalline silicon (Si), andthe third material of the third solar cell is InGaAs.